1. Field of the Invention
The present invention relates to shift hydraulic system of a belt-type continuously variable transmission. More particularly, the present invention relates to control apparatus and method for preventing an occurrence of a belt slip of the belt-type continuously variable transmission (so called, CVT).
2. Description of the Related Art
A Japanese Patent Application First Publication No. Heisei 9-250370 published on Sep. 22, 1997 exemplifies a previously proposed shift hydraulic system for a belt-type continuously variable transmission. In the above-described Japanese Patent Application First Publication, when an electromagnetic hydraulic control valve has failed to control a line pressure, a technique to prevent an excessive rise in the line pressure is disclosed. That is to say, if the electromagnetic hydraulic control valve has failed, the hydraulic valve becomes an open state such that the hydraulic of an oil pump is disabled to be drained and an excessive rise in the line pressure causes a clamp pressure to clamp a belt of the continuously variable transmission to be excessively high. Hence, the frictional load of the belt becomes large, a fuel consumption becomes wasteful, and a durability of the belt becomes worsened.